Type I diabetes, which is an autoimmune disease, is caused by the destruction of pancreatic β cells resulting from abnormal function of the autoimmune system. Pancreatic β cells are a subset of cells composing the islets of Langerhans (pancreatic islets) located in the interstitial tissue of the pancreas, and produce insulin, which is crucial for the maintenance of life, and are involved in the regulation of the in vivo metabolism, for example, of glucose metabolism, protein synthesis, formation and storage of neutral fat, and the like. The destruction of pancreatic β cells causes the lack of insulin in the living body, resulting in abnormalities in glucose tolerance, such as disordered sugar metabolism, whereby blood glucose levels are not maintained in a normal range, and in severe cases, leading to crises of life, such as chronic hyperglycemia, glycosuria, loss of water and electrolytes, ketoacidosis, and coma. In addition, complications for long periods include, for example, neurological disorders, retinopathies, renal disorders, systemic degenerative changes of large or small blood vessels, and infections.
At the present time, the most common and practical therapeutic method for type I diabetes is one by which a pharmaceutical containing insulin or a modified insulin as a main ingredient is administered twice or more per day. Patients who have developed type I diabetes have to continue such a therapeutic method for the rest of their lives and must always have the pharmaceutical at hand, which can be said to be a therapeutic method with extremely poor compliance.
With respect to the mechanism of its development, it is widely recognized from many research reports that based on abnormal function of the suppressive immune system, the development of type I diabetes closely correlates with the activation of CD4-positive T cells and CD8-positive T cells specific for self-antigens derived from the pancreatic islets. In addition to genetic predispositions, infection with certain viruses and exposure to environmental factors such as diet trigger a decrease in the number of and abnormal function of regulatory T cells (Treg cells) and natural killer T cells (NKT cells) responsible for antigen-specific immunosuppression. Such abnormal function of the suppressive immune system activates self-antigen-reactive type 1 helper T cells (pathogenic Th1 cells, which are CD4-positive cells), which would normally be subjected to suppression, and further activates self-antigen-specific cytotoxic T cells (CTLs, which are CD8-positive cells), resulting in specific destruction of the pancreatic β cells by CTLs invaded in the pancreatic islets, which causes their irreversible dysfunction.
A number of studies have been made and reported on the basis of the idea that based on the recognition as described above, the induction of the suppressive immune system to self-antigens derived from the pancreatic islets and the suppression of the production of pathogenic Th1 cells and self-antigen-specific CTLs lead to an improvement in the effect of prevention of the onset of type I diabetes and of treatment of type I diabetes.
For example, studies which are intended to improve the effect of prevention of the onset of type I diabetes and of treatment of type I diabetes by returning suppressive immune functions to normal are directed to the induction of self-antigen-specific Treg cells. According to these studies, self-antigen-specific Treg cells are induced by immunization with a certain self-antigen alone or in combination with an adjuvant. The induced Treg cells are then activated in the pancreas or pancreatic lymph nodes expressing the self-antigen, so that the activation of pathogenic Th1 cells is suppressed, while the cytotoxic activity of CTLs which infiltrate in the pancreatic islets and destroy pancreatic β cells is suppressed by by-stander effects. Non-Patent Documents 1 and 3 report that in tests in pre-diabetic NOD mice, insulin and an insulin peptide B:9-23, which are self-antigens, exhibited effects of delaying the progress of diabetes and suppressing its onset when the NOD mice were immunized with these self-antigens. In addition, Patent Document 1 reports that the onset of type I diabetes can be suppressed by administration of the B chain of insulin or a fragment thereof and an adjuvant for enhancing immune responses. Although, as reported, antigen-specific immunosuppression was shown to be effective in preventing the onset of type I diabetes and treating type I diabetes, drugs exerting a satisfactory effect or their clinical application have not been established.
Further, studies which are intended to improve the effect of prevention of the onset of type I diabetes and of treatment of type I diabetes by returning other suppressive immune functions to normal are directed to attempting to accomplish a qualitative and functional improvement of NKT cells. From these studies, it has been known that such an improvement has an effect of delaying the onset of the disease. NKT cells secrete interleukin 4 (IL-4) and IL-10, which are cytokines, whereby they suppress unnecessary activation and abnormal immune responses of other immune cells, so that they regulate functions of the entire immune system to be maintained at normal levels. Pre-diabetes groups with genetic predispositions have been observed to have smaller numbers of NKT cells and reduced amounts of IL-4 production, and it has been suggested that NKT cells may be involved in the onset of diabetes. In this regard, research was made on the prevention of the onset of diabetes by the activation of NKT cells through the administration of a glycolipid, α-galactosylceramide (α-GalCer), which is a ligand of NKT cells. For example, Patent Document 2 indicates that the administration of α-GalCer delays the onset of type I diabetes. However, the administration of α-GalCer exhibited an effect of delaying the onset of the disease, but did not provide a sufficient preventive effect against the disease (see Patent Document 2, and Examples 2 and 3 described below), and thus has not lead to clinical application. Furthermore, Patent Document 2 discloses an example in which α-GalCer was administered into the thymus, but examinations were not performed on what effect the difference in the mode of administration of α-GalCer has on the effect of prevention or treatment of type I diabetes.
These methods in which the onset of type I diabetes is suppressed by administration of insulin or an insulin peptide alone or of α-GalCer alone require continuous administration of these respective agents, and thus do not provide a solution to the problem of frequent administration, which is an obstacle to current treatments with insulin and modified insulin.
On the other hand, it has been proposed to use α-GalCer to prevent the onset of or to treat allergic diseases caused by the production of certain antibodies, as well as autoimmune diseases of the type in which the production of an antibody (autoantibody) is greatly related to the onset of the disease. For example, Patent Document 3 reports that immune diseases such as allergic diseases can be prevented or treated by administration of liposomes containing α-GalCer, whereby the drug is transported into immune cells in the spleen where antibodies are produced, so that the effects of inducing T cells producing IL-10 and of suppressing the production of IgE antibodies are exhibited in a spleen-specific manner. Patent Document 4 reports that autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatism can be prevented or treated with preparations having α-GalCer and a target antigen contained in a liposome, allowing the drug to be transported into immune cells in the spleen, thereby providing an effect of suppressing the production of IgE antibodies resulting from the target antigen and, at the same time, an effect of suppressing the production of autoantibodies resulting from the target antigen. These methods in which α-GalCer is used to prevent or treat autoimmune diseases are expected to be effective for preventing the onset of or treating SLE and rheumatism by virtue of the effect of suppressing the production of antigen-specific autoantibodies. However, in cases of autoimmune diseases of the type in which suppressive immune functions are attenuated as in type I diabetes and the destruction of pancreatic islets due to the activation of pathogenic Th1 cells and self-antigen-specific CTLs resulting from self-antigens or epitopes is greatly associated with the onset of the disease, the suppression of the production of antibodies is generally thought to have no relevance to techniques that improve the effect of prevention or treatment of diabetes (Non-Patent Documents 1 and 2).
As described above, it is important, in preventing the onset of type I diabetes or improving the effect of treatment of type I diabetes, to improve the function of the self-antigen-specific suppressive immune system, and to suppress the activation of pathogenic Th1 cells and to suppress the production of self-antigen-specific CTLs. When such antigen-specific immunotherapies are carried out, the ascertainment of self-antigens or epitopes is very important in improving therapeutic effects. Epitopes stemming from proteins derived from the pancreatic islets have been found in association with type I diabetes: for example, examinations of antibodies in the serum from patients with type I diabetes revealed the production of antibodies directed to three proteins derived from the pancreatic islets, insulin, glutamic acid decarboxylase (GAD65), and islet tyrosine phosphatase (IA-2), and further, search studies for T-cell epitopes found islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP), zinc transporter (ZnT8), and others. Among these, insulin-derived antigens are expected to be a major factor in the onset of type I diabetes, and it has also been found that about 80% of the CD8-positive T-cell epitopes derived from human pancreatic β cells that have been found in the past are derived from proinsulin and insulin (Non-Patent Document 3). For major T-cell epitopes from proinsulin/insulin, a peptide “B:9-23,” which is composed of amino acids 9 to 23 of the B chain in the insulin molecule, was identified in NOD mice (Non-Patent Document 4), and a peptide “B:11-27,” which is composed of amino acids 11 to 27 of the B chain in the insulin molecule was identified in humans (Non-Patent Document 5). In tests using non-obese diabetic (NOD) mouse, which is an animal model for type I diabetes, the deficiency of GAD65 or IA-2 did not suppress the onset of the disease, whereas the deficiency of insulin B:9-23 resulted in complete suppression of its onset. Also from these results, it is suspected that proinsulin-derived epitopes may be deeply involved in the onset of type I diabetes.
In addition, cell/tissue transplantation therapies in which a normal pancreatic islet tissue is transplanted to patients with diabetes are also known as an effective therapy for diabetes. These transplantation therapies are considered to be very effective especially for patients who have difficulty in controlling the level of blood sugar and who are of a type in which the insulin treatment and diet and exercise therapy are not effective and severe hypoglycemic episodes are caused. However, at the same time, there are several problems, for example, in that it is difficult at this time to maintain control of blood sugar levels for an extended period by a single transplantation and thus more than one transplantations have to be performed. For example, Non-Patent Document 6, which is an article written by a group of Edmond A. Ryan et al. in Edmonton, Canada, reports the results of five-year follow-up after islet transplantation, showing that 50 to 70% of the transplants were lost at relatively early stages after the transplantation. This would be attributed to body's responses to the transplant. For example, it is thought that the transplants were injured and lost by activation of the blood coagulation cascade and the complement system, such as platelet aggregation, non-specific inflammatory responses by macrophages, and innate immune responses. Additionally, islet transplants that have escaped from injury at early stages after the transplantation will be damaged by self-antigen-specific CTLs present prior to transplantation in the patients and subjected to progressive damage to the pancreatic islets, making it difficult to maintain long-term control of blood sugar levels. In order to overcome problems in these transplantation therapies and to improve the effect of treatment, it is important to use an anti-inflammatory agent and to develop a new immunosuppressive therapy targeting self-antigen-specific CTLs.
Taking these conventional techniques into consideration as a background, there is a strong need for development of a preparation which is remarkably excellent in the effect of prevention or treatment of type I diabetes and is clinically applicable.